U.S. patent application number 10/000471 was filed with the patent office on 2002-05-09 for electronically tunable rf diplexers tuned by tunable capacitors.
Invention is credited to Shamsaifar, Khosro, Xu, Jian.
Application Number | 20020053954 10/000471 |
Document ID | / |
Family ID | 22920812 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020053954 |
Kind Code |
A1 |
Shamsaifar, Khosro ; et
al. |
May 9, 2002 |
Electronically tunable RF diplexers tuned by tunable capacitors
Abstract
A diplexer includes a first tunable bandpass filter connected to
a first port, a second tunable bandpass filter connected to a
second port, and a coupling element for coupling the first bandpass
filter and the second bandpass filter to a third port. Each of the
tunable bandpass filters includes a tunable capacitor, wherein a
control signal applied to the tunable capacitor controls the
transmission characteristic of the filter. The tunable capacitor
can be a tunable dielectric varactor or a microelectromechanical
variable capacitor. The coupling element can include one of: a
circulator, a T-junction, and an orthomode transducer. Each of the
first and second filters can comprise a fin line filter including a
plurality of tunable dielectric capacitors mounted within a
waveguide for controlling the filter transmission characteristics.
Fixed bandpass filters can be inserted between each of the tunable
bandpass filters and the coupling element.
Inventors: |
Shamsaifar, Khosro;
(Ellicott City, MD) ; Xu, Jian; (Clarksville,
MD) |
Correspondence
Address: |
Robert P. Lenart
Pietragallo, Bosick & Gordon
One Oxford Centre, 38th Floor
301 Grant Street
Pittsburgh
PA
15219
US
|
Family ID: |
22920812 |
Appl. No.: |
10/000471 |
Filed: |
October 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60243962 |
Oct 26, 2000 |
|
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Current U.S.
Class: |
333/135 |
Current CPC
Class: |
H01P 1/213 20130101 |
Class at
Publication: |
333/135 |
International
Class: |
H01P 005/12 |
Claims
What is claimed is:
1. A diplexer comprising: a first tunable bandpass filter including
a first tunable capacitor and connected to a first port; a second
tunable bandpass filter including a second tunable capacitor and
connected to a second port; and means for coupling the first
bandpass filter and the second bandpass filter to a third port.
2. A diplexer according to claim 1, wherein the first and second
tunable capacitors each comprise: a tunable dielectric
varactor.
3. A diplexer according to claim 1, wherein the first and second
tunable capacitors each comprise: a microelectromechanical variable
capacitor.
4. A diplexer according to claim 1, wherein the means for coupling
the first bandpass filter and the second bandpass filter to a third
port comprises one of: a circulator, a T-junction, and an orthomode
transducer.
5. A diplexer according to claim 1, wherein the first tunable
bandpass filter comprises: a first waveguide; and a first septum
position along an axis of the first waveguide; and wherein the
first tunable capacitor is mounted on the septum.
6. A diplexer according to claim 5, wherein the first tunable
capacitor comprises: a substrate having a first dielectric constant
and having generally a planar surface; a tunable dielectric layer
positioned on the generally planar surface of the substrate, the
tunable dielectric layer having a second dielectric constant
greater than said first dielectric constant; and first and second
electrodes positioned on a surface of the tunable dielectric layer
opposite the generally planar surface of the substrate, said first
and second electrodes being separated to form a gap
therebetween.
7. A diplexer according to claim 6, wherein the first tunable
capacitor further comprises: an insulating material in said
gap.
8. A diplexer according to claim 6, wherein the tunable dielectric
layer in the first tunable dielectric varactor has a permittivity
in a range from about 20 to about 2000, and a tunability in a range
from about 10% to about 80% at a bias voltage of about 10
V/.mu.m.
9. A diplexer according to claim 5, wherein the second tunable
bandpass filter comprises: a second waveguide; and a second septum
position along an axis of the second waveguide; and wherein the
second tunable capacitor is mounted on the second septum.
10. A diplexer according to claim 9, wherein the second tunable
capacitor comprises: a substrate having a first dielectric constant
and having generally a planar surface; a tunable dielectric layer
positioned on the generally planar surface of the substrate, the
tunable dielectric layer having a second dielectric constant
greater than said first dielectric constant; and first and second
electrodes positioned on a surface of the tunable dielectric layer
opposite the generally planar surface of the substrate, said first
and second electrodes being separated to form a gap
therebetween.
11. A diplexer according to claim 10, wherein the second tunable
capacitor further comprises: an insulating material in said
gap.
12. A diplexer according to claim 9, wherein the tunable dielectric
layer in the second tunable capacitor has a permittivity in a range
from about 20 to about 2000, and a tunability in a range from about
10% to about 80% at a bias voltage of about 10 V/.mu.m.
13. A diplexer according to claim 10, further comprising: a first
fixed bandpass filter connected between the first tunable bandpass
and the means for coupling the first bandpass filter and the second
bandpass filter to a third port; and a second fixed bandpass filter
connected between the second tunable bandpass and the means for
coupling the first bandpass filter and the second bandpass filter
to a third port.
14. A diplexer according to claim 13, wherein: each of the first
and second fixed bandpass filters has a larger passband than each
of the first and second tunable filters.
15. A diplexer according to claim 13, wherein: the first tunable
filter has a passband that can be tuned within a passband of the
first fixed bandpass filter; and the second tunable filter has a
passband that can be tuned within a passband of the second fixed
bandpass filter.
16. A diplexer according to claim 1, wherein: the first tunable
bandpass filter comprises a first plurality of resonators, wherein
the first tunable capacitor couples a signal between two of the
resonators in the first plurality of resonators; and the second
tunable bandpass filter comprises a second plurality of resonators,
wherein the second tunable capacitor couples a signal between two
of the resonators in the second plurality of resonators.
17. A diplexer according to claim 1, wherein: the first tunable
bandpass filter comprises a first plurality of resonators, wherein
the first tunable capacitor is positioned within one of the
resonators in the first plurality of resonators; and the second
tunable bandpass filter comprises a second plurality of resonators,
wherein the second tunable capacitor is positioned within one of
the resonators in the second plurality of resonators.
Description
CROSS REFERENCE TO A RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/243,962, filed Oct. 26, 2000.
FIELD OF INVENTION
[0002] The present invention generally relates to electronic
diplexers, and more particularly to tunable diplexers.
BACKGROUND OF INVENTION
[0003] Commercially available radio frequency (RF) diplexers
include two fixed bandpass filters sharing a common port (antenna
port) through a circulator or a T-junction. Signals applied to the
antenna port are coupled to a receiver port through the receive
bandpass filter, and signals applied to a transmitter port will
reach the antenna port through a transmit filter. The receive port
and transmitter port are isolated from each other due to the
presence of the filters and the circulator, or T-junction. In
another configuration, the receive signals reaching the antenna
will be divided into two sub-bands according to the band pass
frequencies of the filters. In the opposite direction, two signals
reaching the non-common ports of the filters will be combined and
output at the common port. Also in this configuration the two
filters are isolated with respect to each other.
[0004] Fixed diplexers are commonly used in point-to-point and
point-to-multipoint radios where two-way communication enables
voice, video and data traffic within the RF frequency range. Fixed
diplexers need to be wide band so that their count does not exceed
reasonable numbers to cover the desired frequency plan.
[0005] It would be desirable to have a tunable diplexer that would
could be used to replace fixed diplexers in receivers. A single
tunable diplexer solution would enable radio manufacturers to
replace several fixed diplexers covering adjacent frequencies. This
versatility can provide front end RF tunability in real time
applications and decrease deployment and maintenance costs through
software controls and reduced component count.
SUMMARY OF THE INVENTION
[0006] Diplexers constructed in accordance with this invention
include a first tunable bandpass filter connected to a first port,
a second tunable bandpass filter connected to a second port, and a
coupling element for coupling the first bandpass filter and the
second bandpass filter to a third port. Each of the tunable
bandpass filters includes at least one tunable capacitor, wherein a
control signal applied to the tunable capacitor controls the
transmission characteristic of the filter. The tunable capacitor
can be a tunable dielectric varactor or a microelectromechanical
variable capacitor. The coupling element can include one of: a
circulator, a T-junction, and an orthomode transducer. Each of the
first and second filters can comprise a fin line filter including a
plurality of tunable dielectric capacitors mounted within a
waveguide for controlling the filter transmission characteristics.
Fixed bandpass filters can be inserted between each of the tunable
bandpass filters and the coupling element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic representation of a tunable diplexer
constructed in accordance with this invention;
[0008] FIG. 2 is a graph of the frequency response of one of the
filters of the diplexer of FIG. 1;
[0009] FIG. 3 is a schematic representation of another tunable
diplexer constructed in accordance with this invention;
[0010] FIG. 4 is a schematic representation of another tunable
diplexer constructed in accordance with this invention;
[0011] FIG. 5 is a schematic representation of a filter that can be
used in the diplexers of FIGS. 1, 3 or 4;
[0012] FIG. 6 is a cross sectional view of another fin line filter
that can be used in the diplexers of FIGS. 1, 3 or 4;
[0013] FIG. 7 is a top view of a tunable dielectric capacitor that
can be used in the filter of FIG. 5 or 6;
[0014] FIG. 8 is a cross-sectional view of the tunable dielectric
capacitor of FIG. 7 taken along line 8-8;
[0015] FIG. 9 is a graph illustrating the properties of the tunable
dielectric capacitor of FIGS. 7 and 8;
[0016] FIG. 10 is a graph illustrating the frequency response of an
electronically tunable diplexer constructed in accordance with this
invention for operation in the K-band with overall unloaded Q of
450 under zero bias conditions;
[0017] FIG. 11 is a graph illustrating the frequency response of an
electronically tunable diplexer constructed in accordance with this
invention for operation in K-band with overall unloaded Q of 400
under full bias conditions;
[0018] FIG. 12 is a schematic representation of another tunable
diplexer constructed in accordance with this invention; and
[0019] FIGS. 13 and 14 are graphs illustrating the properties of
the tunable and fixed bandpass filters of the diplexer of FIG.
12.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention provides tunable diplexers having low
insertion loss, fast tuning speed, high power-handling capability,
high IP3 and low cost in the microwave frequency range.
[0021] Referring to the drawings, FIG. 1 is a schematic
representation of a tunable diplexer 10 constructed in accordance
with this invention. The tunable diplexer 10 includes two
electronically tunable bandpass filters 12 and 14 connected to a
common port 16 through a coupling means 18. In the embodiment of
FIG. 1, the coupling means is a circulator 20. Filter 12 is a
receive filter connected to couple signals from the coupling means
to a first (receive) port 22. Filter 14 is a transmit filter
connected to couple signals from the coupling means to a second
(transmit) port 24. Filters 12 and 14 are tunable bandpass filters.
In the preferred embodiment, the filters include tunable dielectric
varactors that can be rapidly tuned and are used to control the
transmission characteristics of the filters. Alternatively,
microelectromechanical (MEM) variable capacitors can be used in the
tunable filters. A control unit 26, which can be a computer or
other processor, is used to supply a control signal to tunable
capacitors in the filters, preferably through high impedance
control lines. The control unit can use an open loop or closed loop
control technique. Various types of tunable filters can be used in
the diplexers of this invention. The circulator 20 of FIG. 1
achieves isolation between the two filters.
[0022] FIG. 2 is a graph of the frequency response of one of the
filters of the diplexer of FIG. 1. The circulator provides -25 dB
of isolation 28. Curve 30 represents the filter passband when
tunable dielectric varactors in the filters are biased at a first
level, which can be zero volts, and curve 32 represents the filter
passband when the varactors are biased at a second level, such as
300 volts. The control unit can be used to control the bias voltage
on varactors in the filters and thereby control the passband of the
filters.
[0023] FIG. 3 is a schematic representation of another tunable
diplexer 40 constructed in accordance with this invention. Diplexer
40 uses a T-junction 42 as the coupling element 18.
[0024] FIG. 4 is a schematic representation of another tunable
diplexer 44 constructed in accordance with this invention. Diplexer
44 uses an Ortho-Mode Transducer (OMT) 46 as the coupling element
18.
[0025] One possible structure for the filters is a fin line filter,
which includes a rectangular waveguide cut in two halves according
to the E-plane, plus an e-plane metal septum. FIG. 5 is a schematic
representation of a two-pole fin line filter 50 that can be used in
the diplexers of FIGS. 1, 3 or 4. The filter includes a rectangular
waveguide 52 and a septum 54 mounted along an axis 56 of the
waveguide. The septum is divided into three sections 58, 60 and 62.
A longitudinal slot 64 passes into each of the other sections.
Tunable capacitors 66, 68, 70 and 72 are mounted across the gaps in
the septum. The tunable capacitors can be microelectromechanical
variable capacitors or tunable dielectric varactors. By applying a
tuning voltage to the varactors, the passband of the filter can be
changed.
[0026] FIG. 6 is a cross sectional view of another tunable fin line
filter 88 that can be used in the diplexers of FIGS. 1, 3 or 4. The
filter 88 includes four tunable dielectric varactors on a
symmetrical fin line in a rectangular waveguide. An electrically
tunable filter is achieved at room temperature by mounting several
tunable dielectric varactors on a fin line waveguide. The fin line
construction is comprised of three foil copper plates 90, 92 and 94
with thickness of 0.2 mm placed at the center of the waveguide 96
along its longitudinal axis. Two lateral plates with shorted end
fin line resonators 98 and 100 are grounded due to the contact with
the waveguide. Central plate 92 is insulated for DC voltage from
the waveguide by mica 102 and 104 and is used to apply the control
voltage to the tunable capacitors 106, 108, 110 and 112. The
tunable dielectric varactors are soldered in the end of the fin
line resonators between plates 90 and 92, and plates 94 and 92.
Flanges 114 and 116 support the plates.
[0027] FIGS. 7 and 8 are top and cross sectional views of a tunable
dielectric varactor 100 that can be used in the tunable bandpass
filters of this invention. The varactor 100 includes a substrate
102 having a generally planar top surface 104. A tunable dielectric
layer 106 is positioned adjacent to the top surface of the
substrate. A pair of metal electrodes 108 and 110 are positioned on
top of the ferroelectric layer. The substrate 102 is comprised of a
material having a relatively low permittivity such as MgO, Alumina,
LaAlO.sub.3, Sapphire, or a ceramic. For the purposes of this
description, a low permittivity is a permittivity of less than
about 30. The tunable dielectric layer 106 is comprised of a
material having a permittivity in a range from about 20 to about
2000, and having a tunability in the range from about 10% to about
80% at a bias voltage of about 10 V/.mu.m. In the preferred
embodiment this layer is preferably comprised of Barium-Strontium
Titanate, Ba.sub.xSr.sub.1-xTiO.sub.3 (BSTO), where x can range
from zero to one, or BSTO-composite ceramics. Examples of such BSTO
composites include, but are not limited to: BSTO-MgO,
BSTO-MgAl.sub.2O.sub.4, BSTO-CaTiO.sub.3, BSTO-MgTiO.sub.3,
BSTO-MgSrZrTiO.sub.6, and combinations thereof. The tunable layer
in one preferred embodiment has a dielectric permittivity greater
than 100 when subjected to typical DC bias voltages, for example,
voltages ranging from about 5 volts to about 300 volts. A gap 112
of width g, is formed between the electrodes 108 and 110. The gap
width must be optimized to increase ratio of the maximum
capacitance C.sub.max to the minimum capacitance C.sub.min
(C.sub.max/C.sub.min) and increase the quality facto (Q) of the
device. The optimal width, g, will be determined by the width at
which the device has maximum C.sub.max/C.sub.min and minimal loss
tangent.
[0028] A controllable voltage source 114 is connected by lines 116
and 118 to electrodes 108 and 110. This voltage source is used to
supply a DC bias voltage to the tunable dielectric layer, thereby
controlling the permittivity of the layer. The varactor also
includes an RF input 120 and an RF output 122. The RF input and
output are connected to electrodes 108 and 110, respectively, by
soldered or bonded connections.
[0029] In the preferred embodiments, the varactors may use gap
widths of less than 5-50 .mu.m. The thickness of the tunable
dielectric layer ranges from about 0.1 .mu.m to about 20 .mu.m. A
sealant 124 can be positioned within the gap and can be any
non-conducting material with a high dielectric breakdown strength
to allow the application of high voltage without arcing across the
gap. In one embodiment, the sealant can be epoxy or
polyurethane.
[0030] The other dimension that strongly influences the design of
the varactors is the length, L, of the gap as shown in FIG. 7. The
length of the gap L can be adjusted by changing the length of the
ends 126 and 128 of the electrodes. Variations in the length have a
strong effect on the capacitance of the varactor. The gap length
will optimized for this parameter. Once the gap width has been
selected, the capacitance becomes a linear function of the length
L. For a desired capacitance, the length L can be determined
experimentally, or through computer simulation.
[0031] The electrodes may be fabricated in any geometry or shape
containing a gap of predetermined width. The required current for
manipulation of the capacitance of the varactors disclosed in this
invention is typically less than 1 .mu.A. In the preferred
embodiment, the electrode material is gold. However, other
conductors such as copper, silver or aluminum, may also be used.
Gold is resistant to corrosion and can be readily bonded to the RF
input and output. Copper provides high conductivity, and would
typically be coated with gold for bonding or nickel for
soldering.
[0032] FIGS. 7 and 8 show a voltage tunable planar varactor having
a planar electrode with a predetermined gap distance on a single
layer tunable bulk, thick film or thin film dielectric. The applied
voltage produces an electric field across the gap of the tunable
dielectric that produces an overall change in the capacitance of
the varactor. The width of the gap can range from 5 to 50 .mu.m
depending on the performance requirements.
[0033] By employing the diplexer topology of this invention, a
diplexer with receive frequency of, for example, 21.186 GHz and
transmit frequency of 22.356 GHz at zero DC field could be tuned to
receive frequency of 21.732 GHz and transmit frequency of 22.887
GHz at a bias electric field of 15 V/.mu.m. All other frequencies
between these two values can be covered by applying an electric
field strength of 0 to 15 V/.mu.m.
[0034] Additional description of the fin line filter of FIG. 6 and
the tunable dielectric varactor of FIGS. 7 and 8, can be found in
U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999,
which is hereby incorporated by reference.
[0035] FIG. 9 shows an example of the capacitance 130 and the loss
tangent 132 of a tunable dielectric varactor. By applying voltage
to the varactor its capacitance value changes and consequently the
frequency of the diplexer will be varied.
[0036] FIGS. 10 and 11 show measured frequency responses of the
tunable diplexer with different bias voltages on the tunable
dielectric varactors. Curves 134 and 136 of FIG. 10 illustrate an
example frequency response of one of the tunable filters having
tunable dielectric varactors operated at different varactor control
voltages. Curves 138 and 140 of FIG. 10 illustrate an example
frequency response of another one of the tunable filters having
tunable dielectric varactors operated at different varactor control
voltages. It is observed that with this structure a tunability of
about 540 MHz is achieved without a considerable degradation of the
diplexer response.
[0037] While a fin line filter has been described, other structures
for the filter, such as iris coupled or inductive post coupled
waveguide cavity filters, or filters based on dielectric resonator
cavities, or other resonators such as lumped element LC circuits,
or planar structure resonators such as microstrip, stripline or
coplanar resonators, etc. can be used in the diplexers of this
invention. Variation of the capacitance of the tunable dielectric
varactors in the tunable filters affects the resonant frequency of
filter sections, and therefore affects the passband of the filters.
Inherent in every electronically tunable radio frequency filter is
the ability to rapidly tune the response using high-impedance
control lines. Tunable dielectric materials technology enables
these tuning properties, as well as, high Q values, low losses and
extremely high IP3 characteristics, even at high frequencies.
[0038] When using the T-junction, the required isolation between
transmit and receive will be provided by the filters, which will
need a large number of poles in many practical applications.
Obviously, a large number of poles means a large insertion loss. In
order to reduce insertion loss while maintaining the necessary
isolation, fixed bandpass filters can be inserted between the
tunable filters and the coupling element. FIG. 12 is a schematic
representation of another tunable diplexer constructed in
accordance with this invention that includes fixed bandpass
filters.
[0039] FIG. 12 is a schematic representation of a tunable diplexer
150 constructed in accordance with this invention. The tunable
diplexer 150 includes two electronically tunable bandpass filters
152 and 154 having bandpass characteristics that can be varied by
applying a control signal from the control unit 156 to tunable
capacitors in the filters. A coupling element in the form of a
T-junction 158 receives signals from a fixed bandpass filter 160
that is connected the tunable filter 158, and passes signals to a
fixed filter 162 that is connected the tunable filter 154. An
antenna can be connected to the T-junction through line 164.
Tunable filter 154 passes received signals to a receiver on line
166. Tunable filter 152 receives signals to be transmitted on line
168. The filters can include tunable dielectric varactors or MEMS
tunable capacitors that can be rapidly tuned and are used to
control the transmission characteristics of the filters.
[0040] FIGS. 13 and 14 are graphs illustrating the properties of
the tunable and fixed bandpass filters of the diplexer of FIG. 12.
In one example, the fixed filter is a 6-pole wide bandwidth filter
having the passband illustrated by curve 170 of FIG. 13. The
tunable filter has only two poles for low insertion loss, and is
narrow band, having a passband that can be tuned as illustrated by
curves 174 and 176 of FIG. 13. This results in a filter tuning
range illustrated by item 176 in FIG. 13. By using the combination
of fixed an tunable filters, the losses are kept within the
specification while the required isolation is achieved. Because the
tunable filter is a narrow band filter, the superposition of the
two filters will have the desired narrow band response as
illustrated by curves 178 and 180 of FIG. 14. The overall response
is essentially the bandwidth of the tunable filter.
[0041] One possible structure for the filters is a finline filter
as described above having a rectangular waveguide cut in two halves
according to the E-plane, plus an e-plane metal septum, with
tunable varactors are mounted on the septum. Other structures for
the filter, such as iris coupled or inductive post coupled
waveguide cavity filters, or filters based on dielectric resonator
cavities, etc. are also possible. Also, where the varactors are
positioned inside the resonant cavity, other tunable capacitor
structures can be used. Variation of the capacitance of the tunable
capacitor affects the distribution of the electric filed inside the
cavity, which in turn varies the resonant frequency.
[0042] The electronically tunable filters have low insertion loss,
fast tuning speed, high power-handling capability, high IP3 and low
cost in the microwave frequency range. Compared to the
voltage-controlled semiconductor diode varactors,
voltage-controlled tunable dielectric capacitors have higher Q
factors, higher power-handling and higher IP3. Voltage-controlled
tunable dielectric capacitors have a capacitance that varies
approximately linearly with applied voltage and can achieve a wider
range of capacitance values than is possible with semiconductor
diode varactors. The tunable dielectric varactor based tunable
diplexers of this invention have the merits of lower loss, higher
power-handling, and higher IP3, especially at higher frequencies
(>10 GHz).
[0043] The tunable dielectric varactors in the preferred embodiment
of the present invention can include a low loss
(Ba,Sr)TiO.sub.3-based composite film. The typical Q factor of the
tunable dielectric capacitors is 200 to 500 at 2 GHz, and 50 to 100
at 20 to 30 GHz, with a capacitance ratio (C.sub.max/C.sub.min),
which is independent of frequency, of around 2. A wide range of
capacitance of the tunable dielectric capacitors is variable, say
0.1 pF to 10 pF. The tuning speed of the tunable dielectric
capacitor is less than 30 ns. The practical tuning speed is
determined by auxiliary bias circuits.
[0044] Tunable dielectric materials have been described in several
patents. Barium strontium titanate (BaTiO.sub.3--SrTiO.sub.3), also
referred to as BSTO, is used for its high dielectric constant
(200-6,000) and large change in dielectric constant with applied
voltage (25-75 percent with a field of 2 Volts/micron). Tunable
dielectric materials including barium strontium titanate are
disclosed in U.S. Pat. No. 5,427,988 by Sengupta, et al. entitled
"Ceramic Ferroelectric Composite Material-BSTO-MgO"; U.S. Pat. No.
5,635,434 by Sengupta, et al. entitled "Ceramic Ferroelectric
Composite Material-BSTO-Magnesium Based Compound"; U.S. Pat. No.
5,830,591 by Sengupta, et al. entitled "Multilayered Ferroelectric
Composite Waveguides"; U.S. Pat. No. 5,846,893 by Sengupta, et al.
entitled "Thin Film Ferroelectric Composites and Method of Making";
U.S. Pat. No. 5,766,697 by Sengupta, et al. entitled "Method of
Making Thin Film Composites"; U.S. Pat. No. 5,693,429 by Sengupta,
et al. entitled "Electronically Graded Multilayer Ferroelectric
Composites"; U.S. Pat. No. 5,635,433 by Sengupta entitled "Ceramic
Ferroelectric Composite Material BSTO-ZnO"; U.S. Pat. No. 6,074,971
by Chiu et al. entitled "Ceramic Ferroelectric Composite Materials
with Enhanced Electronic Properties BSTO-Mg Based Compound-Rare
Earth Oxide". These patents are incorporated herein by
reference.
[0045] Barium strontium titanate of the formula
Ba.sub.xSr.sub.1-xTiO.sub.- 3 is a preferred electronically tunable
dielectric material due to its favorable tuning characteristics,
low Curie temperatures and low microwave loss properties. In the
formula Ba.sub.xSr.sub.1-xTiO.sub.3, x can be any value from 0 to
1, preferably from about 0.15 to about 0.6. More preferably, x is
from 0.3 to 0.6.
[0046] Other electronically tunable dielectric materials may be
used partially or entirely in place of barium strontium titanate.
An example is Ba.sub.xCa.sub.1-xTiO.sub.3, where x is in a range
from about 0.2 to about 0.8, preferably from about 0.4 to about
0.6. Additional electronically tunable ferroelectrics include
Pb.sub.xZr.sub.1-xTiO.sub.3 (PZT) where x ranges from about 0.0 to
about 1.0, Pb.sub.xZr.sub.1-xSrTiO- .sub.3 where x ranges from
about 0.05 to about 0.4, KTa.sub.xNb.sub.1-xO.sub.3 where x ranges
from about 0.0 to about 1.0, lead lanthanum zirconium titanate
(PLZT), PbTiO.sub.3, BaCaZrTiO.sub.3, NaNO.sub.3, KNbO.sub.3,
LiNbO.sub.3, LiTaO.sub.3, PbNb.sub.2O.sub.6, PbTa.sub.2O.sub.6,
KSr(NbO.sub.3) and NaBa.sub.2(NbO.sub.3).sub.5KH.sub.2- PO.sub.4,
and mixtures and compositions thereof. Also, these materials can be
combined with low loss dielectric materials, such as magnesium
oxide (MgO), aluminum oxide (Al.sub.2O.sub.3), and zirconium oxide
(ZrO.sub.2), and/or with additional doping elements, such as
manganese (MN), iron (Fe), and tungsten (W), or with other alkali
earth metal oxides (i.e. calcium oxide, etc.), transition metal
oxides, silicates, niobates, tantalates, aluminates, zirconnates,
and titanates to further reduce the dielectric loss.
[0047] In addition, the following U.S. patent applications,
assigned to the assignee of this application, disclose additional
examples of tunable dielectric materials: U.S. application Ser. No.
09/594,837 filed Jun. 15, 2000, entitled "Electronically Tunable
Ceramic Materials Including Tunable Dielectric and Metal Silicate
Phases"; U.S. application Ser. No. 09/768,690 filed Jan. 24, 2001,
entitled "Electronically Tunable, Low-Loss Ceramic Materials
Including a Tunable Dielectric Phase and Multiple Metal Oxide
Phases"; U.S. application Ser. No. 09/882,605 filed Jun. 15, 2001,
entitled "Electronically Tunable Dielectric Composite Thick Films
And Methods Of Making Same"; U.S. application Ser. No. 09/834,327
filed Apr. 13, 2001, entitled "Strain-Relieved Tunable Dielectric
Thin Films"; and U.S. Provisional Application Ser. No. 60/295,046
filed Jun. 1, 2001 entitled "Tunable Dielectric Compositions
Including Low Loss Glass Frits". These patent applications are
incorporated herein by reference.
[0048] The tunable dielectric materials can also be combined with
one or more non-tunable dielectric materials. The non-tunable
phase(s) may include MgO, MgAl.sub.2O.sub.4, MgTiO.sub.3,
Mg.sub.2SiO.sub.4, CaSiO.sub.3, MgSrZrTiO.sub.6, CaTiO.sub.3,
Al.sub.2O.sub.3, SiO.sub.2 and/or other metal silicates such as
BaSiO.sub.3 and SrSiO.sub.3. The non-tunable dielectric phases may
be any combination of the above, e.g., MgO combined with
MgTiO.sub.3, MgO combined with MgSrZrTiO.sub.6, MgO combined with
Mg.sub.2SiO.sub.4, MgO combined with Mg.sub.2SiO.sub.4,
Mg.sub.2SiO.sub.4 combined with CaTiO.sub.3 and the like.
[0049] Additional minor additives in amounts of from about 0.1 to
about 5 weight percent can be added to the composites to
additionally improve the electronic properties of the films. These
minor additives include oxides such as zirconnates, tannates, rare
earths, niobates and tantalates. For example, the minor additives
may include CaZrO.sub.3, BaZrO.sub.3, SrZrO.sub.3, BaSnO.sub.3,
CaSnO.sub.3, MgSnO.sub.3, Bi.sub.2O.sub.3/2SnO.sub.2,
Nd.sub.2O.sub.3, Pr.sub.7O.sub.11, Yb.sub.2O.sub.3,
Ho.sub.2O.sub.3, La.sub.2O.sub.3, MgNb.sub.2O.sub.6,
SrNb.sub.2O.sub.6, BaNb.sub.2O.sub.6, MgTa.sub.2O.sub.6,
BaTa.sub.2O.sub.6 and Ta.sub.2O.sub.3.
[0050] Thick films of tunable dielectric composites can comprise
Ba.sub.1-xSr.sub.xTiO.sub.3, where x is from 0.3 to 0.7 in
combination with at least one non-tunable dielectric phase selected
from MgO, MgTiO.sub.3, MgZrO.sub.3, MgSrZrTiO.sub.6,
Mg.sub.2SiO.sub.4, CaSiO.sub.3, MgAl.sub.2O.sub.4, CaTiO.sub.3,
Al.sub.2O.sub.3, SiO.sub.2, BaSiO.sub.3 and SrSiO.sub.3. These
compositions can be BSTO and one of these components or two or more
of these components in quantities from 0.25 weight percent to 80
weight percent with BSTO weight ratios of 99.75 weight percent to
20 weight percent.
[0051] The electronically tunable materials can also include at
least one metal silicate phase. The metal silicates may include
metals from Group 2A of the Periodic Table, i.e., Be, Mg, Ca, Sr,
Ba and Ra, preferably Mg, Ca, Sr and Ba. Preferred metal silicates
include Mg.sub.2SiO.sub.4, CaSiO.sub.3, BaSiO.sub.3 and
SrSiO.sub.3. In addition to Group 2A metals, the present metal
silicates may include metals from Group 1A, i.e., Li, Na, K, Rb, Cs
and Fr, preferably Li, Na and K. For example, such metal silicates
may include sodium silicates such as Na.sub.2SiO.sub.3 and
NaSiO.sub.3-5H.sub.2O, and lithium-containing silicates such as
LiAlSiO.sub.4, Li.sub.2SiO.sub.3 and Li.sub.4SiO.sub.4. Metals from
Groups 3A, 4A and some transition metals of the Periodic Table may
also be suitable constituents of the metal silicate phase.
Additional metal silicates may include Al.sub.2Si.sub.2O.sub.7,
ZrSiO.sub.4, KalSi.sub.3O.sub.8, NaAlSi.sub.3O.sub.8,
CaAl.sub.2Si.sub.2O.sub.8, CaMgSi.sub.2O.sub.6, BaTiSi.sub.3O.sub.9
and Zn.sub.2SiO.sub.4. The above tunable materials can be tuned at
room temperature by controlling an electric field that is applied
across the materials.
[0052] In addition to the electronically tunable dielectric phase,
the electronically tunable materials can include at least two
additional metal oxide phases. The additional metal oxides may
include metals from Group 2A of the Periodic Table, i.e., Mg, Ca,
Sr, Ba, Be and Ra, preferably Mg, Ca, Sr and Ba. The additional
metal oxides may also include metals from Group 1A, i.e., Li, Na,
K, Rb, Cs and Fr, preferably Li, Na and K. Metals from other Groups
of the Periodic Table may also be suitable constituents of the
metal oxide phases. For example, refractory metals such as Ti, V,
Cr, Mn, Zr, Nb, Mo, Hf, Ta and W may be used. Furthermore, metals
such as Al, Si, Sn, Pb and Bi may be used. In addition, the metal
oxide phases may comprise rare earth metals such as Sc, Y, La, Ce,
Pr, Nd and the like.
[0053] The additional metal oxides may include, for example,
zirconnates, silicates, titanates, aluminates, stannates, niobates,
tantalates and rare earth oxides. Preferred additional metal oxides
include Mg.sub.2SiO.sub.4, MgO, CaTiO.sub.3, MgZrSrTiO.sub.6,
MgTiO.sub.3, MgAl.sub.2O.sub.4, WO.sub.3, SnTiO.sub.4, ZrTiO.sub.4,
CaSiO.sub.3, CaSnO.sub.3, CaWO.sub.4, CaZrO.sub.3,
MgTa.sub.2O.sub.6, MgZrO.sub.3, MnO.sub.2, PbO, Bi.sub.2O.sub.3 and
La.sub.2O.sub.3. Particularly preferred additional metal oxides
include Mg.sub.2SiO.sub.4, MgO, CaTiO.sub.3, MgZrSrTiO.sub.6,
MgTiO.sub.3, MgAl.sub.2O.sub.4, MgTa.sub.2O.sub.6 and
MgZrO.sub.3.
[0054] The additional metal oxide phases are typically present in
total amounts of from about 1 to about 80 weight percent of the
material, preferably from about 3 to about 65 weight percent, and
more preferably from about 5 to about 60 weight percent. In one
preferred embodiment, the additional metal oxides comprise from
about 10 to about 50 total weight percent of the material. The
individual amount of each additional metal oxide may be adjusted to
provide the desired properties. Where two additional metal oxides
are used, their weight ratios may vary, for example, from about
1:100 to about 100:1, typically from about 1:10 to about 10:1 or
from about 1:5 to about 5:1. Although metal oxides in total amounts
of from 1 to 80 weight percent are typically used, smaller additive
amounts of from 0.01 to 1 weight percent may be used for some
applications.
[0055] In one embodiment, the additional metal oxide phases may
include at least two Mg-containing compounds. In addition to the
multiple Mg-containing compounds, the material may optionally
include Mg-free compounds, for example, oxides of metals selected
from Si, Ca, Zr, Ti, Al and/or rare earths. In another embodiment,
the additional metal oxide phases may include a single
Mg-containing compound and at least one Mg-free compound, for
example, oxides of metals selected from Si, Ca, Zr, Ti, Al and/or
rare earths. The high Q tunable dielectric capacitor utilizes low
loss tunable substrates or films.
[0056] To construct a tunable device, the tunable dielectric
material can be deposited onto a low loss substrate. In some
instances, such as where thin film devices are used, a buffer layer
of tunable material, having the same composition as a main tunable
layer, or having a different composition can be inserted between
the substrate and the main tunable layer. The low loss dielectric
substrate can include magnesium oxide (MgO), aluminum oxide
(Al.sub.2O.sub.3), and lanthium oxide (LaAl.sub.2O.sub.3).
[0057] This invention provides electronically tunable radio
frequency diplexers particularly applicable to microwave radio
applications. Compared to mechanically and magnetically tunable
diplexers, electronically tunable diplexers have the most important
advantage of fast tuning capability over wide band application.
Because of this advantage, they can be used in the applications
such as LMDS (local multipoint distribution service), PCS (personal
communication system), frequency hopping, satellite communication,
and radar systems. Electronically tunable radio frequency diplexers
offer service providers flexibility and scalability never before
accessible. A single diplexer solution enables radio manufacturers
to replace several fixed diplexers covering adjacent frequencies.
This versatility provides front end RF tunability in real time
applications and decreases deployment and maintenance costs through
software controls and reduced component count. Also, fixed
diplexers need to be wide band so that their count does not exceed
reasonable numbers to cover the desired frequency plan. Tunable
diplexers, however, are narrow band, but they can cover even larger
frequency band than fixed diplexers by tuning the filters over a
wide range. Additionally, narrowband filters at the front end are
appreciated from the systems point of view, because they provide
better selectivity and help reduce interference from nearby
transmitters. Narrowband electronically tunable radio frequency
diplexers solutions are also possible for tunable channel
selectivity.
[0058] The preferred embodiment of the invention uses a waveguide
structure, which is tuned by voltage-controlled tunable dielectric
capacitors placed inside the waveguide. In the filter structure,
the tuning element is a voltage-controlled tunable capacitor, which
is made from tunable dielectric material. Since the tunable
capacitors show high Q, high IP3 (low inter-modulation distortion)
and low cost, the tunable diplexer in the present invention has the
advantage of low insertion loss, fast tuning speed, and high power
handling. The present tunable dielectric material technology makes
electronically tunable diplexers very promising in the contemporary
communication system applications.
[0059] Compared to voltage-controlled semiconductor diode
varactors, voltage-controlled tunable dielectric capacitors have
higher Q factors, higher power-handling and higher IP3.
Voltage-controlled tunable dielectric capacitors are employed in
the diplexer structure to achieve the goal of this object. Also,
tunable diplexers based on MEM technology can be used for these
applications. Compared to semiconductor varactor based tunable
diplexers, dielectric varactor based tunable diplexers have the
merits of lower loss, higher power-handling, and higher IP3,
especially at higher frequencies (>10 GHz). MEM based varactors
can also be used for this purpose. They use different bias voltages
to vary the electrostatic force between two parallel plates of the
varactor and hence change its capacitance value. They show lower Q
than dielectric varactors, but can be used successfully for low
frequency applications.
[0060] At least two microelectromachanical variable capacitor
topologies can be used, parallel plate and interdigital. In
parallel plate structure, one of the plates is suspended at a
distance from the other plate by suspension springs. This distance
can vary in response to electrostatic force between two parallel
plates induced by applied bias voltage. In the interdigital
configuration, the effective area of the capacitor is varied by
moving the fingers comprising the capacitor in and out and changing
its capacitance value. MEM varactors have lower Q than their
dielectric counterpart, especially at higher frequencies, but can
be used in low frequency applications.
[0061] Accordingly, the present invention, by utilizing the unique
application of high Q tunable capacitors, provides a high
performance microwave electronically tunable diplexer. While the
present invention has been described in terms of its preferred
embodiments, it will be apparent to those skilled in the art that
various changes can be made to the disclosed embodiments without
departing from the scope of the invention as set forth in the
following claims.
* * * * *